Abstract
The industrial drive for this research is to find new phosphors for application in mercury-free fluorescent lamps and plasma display panels. The vacuum-ultraviolet (VUV) excitation light that is used in these devices allows for the use of phosphors that show emission of two photons for each photon absorbed (quantum cutting).
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The fundamental challenges of this research are to explore the relatively new area of VUV spectroscopy and to design and study new quantum-cutting phosphors.
After an introduction in chapter 1, chapter 2 describes the comparison of high-resolution 4fn-15d → 4fn VUV emission spectra of lanthanides doped in LiYF4 and YPO4 to calculated emission spectra. A good agreement between the intensities and locations of the zero-phonon lines is found.
Chapter 3 deals with the temperature-dependent spectroscopic properties of the Gd-Eu couple in LiGdF4:Eu3+ 0.5%. The decay curves measured at several temperatures can be explained with the current model.
In chapter 4 energy transfer between Pr3+ and Eu3+ is reinvestigated. In agreement with previous results, the experiments presented in this chapter confirm a lack of energy transfer, but additionally a strong quenching of the Pr3+ 1S0 emission by Eu3+ is observed. A low-energy metal-to-metal charge-transfer state (Pr4+-Eu2+) for Pr-Eu pairs is proposed to explain the quenching. When Yb3+ is used as a co-dopant instead of Eu3+, quenching is observed as well, which supports the proposed quenching mechanism.
In chapter 5 energy transfer from Pr3+ to Mn2+, which is expected due to strong spectral overlap, is studied in a number of fluoride host lattices. Surprisingly, energy transfer is not observed. In order to explain the absence of energy transfer, the rates for several mechanisms are calculated. The radiative decay rate of the 1S0 level is indeed larger than the calculated energy-transfer rates, but the complete absence of energy transfer could not be explained. Possible explanations are discussed.
In chapter 6 the focus is shifted towards quantum cutting in three-ion systems
via a new mechanism. For the system YbxY1-xPO4:Tb3+ 1%, experimental evidence for cooperative energy transfer is presented. Excitation and emission spectra provide proof of energy transfer from Tb3+ to Yb3+. Based on the good agreement between experimental and simulated decay curves, it is concluded that energy transfer proceeds exclusively through cooperative dipole-dipole interaction. The energy-transfer rate to two nearest-neighbor Yb3+ ions is 0.26 ms-1. This corresponds to an upper limit for the energy-transfer efficiency of 88% in YbPO4:Tb3+ 1%.
Due to the present work and the efforts of other research groups, it has become clear that the requirements for efficient quantum cutting in a competitive Xe fluorescent tube are not met. However, this research opens new opportunities to increase the efficiency of crystalline Si solar cells. The energy conversion efficiency can be increased by the use of visible-to-infrared quantum-cutting phosphors that convert the visible part of the solar spectrum (without energy loss!) into infrared radiation. A first attempt to realize a visible-to-infrared quantum-cutting phosphor is described in chapter 6 of this thesis.
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